1. Field of the Invention
This invention pertains to electrical contact materials for use in switches and molded case circuit breakers and, more particularly, it pertains to graphite fibers in a silver matrix.
2. Description of the Prior Art
Circuit breakers include electrical contacts that make, carry, and break electrical circuits passing through the circuit breaker. The contacts are made of either elemental metal, composites, or alloys that are derived by the metal-cast method or manufactured by powder metallurgy processes. The ideal metal or metal combination that can function as a perfect contact material under all conditions does not exist. Therefore, an evaluation and understanding of the operating conditions of an electrical contact device including economic considerations is necessary before selecting the most suitable contact material.
Historically, contact materials have consisted almost entirely of silver, silver alloys, and powder metallurgically sintered combinations. Exceptions include some beryllium copper, phosphor bronze, and nickel materials that are also used as contacts. Silver-type contacts, include the pure metal, alloys, and metal powder combinations comprise the majority of contact applications in the electrical industry. Other types of contacts used include platinum group metals, tungsten, molybdenum, copper, copper alloys, and mercury. For more information on electrical contact materials, reference is made to "Electrical-Contact Materials" in volume 3 of the 9th edition of METALS HANDBOOK, published by the American Society for Metals.
Powder metallurgy facilitates combinations of silver as well as copper with other metals. These diverse combinations ordinarily cannot be achieved by alloying. When silver is combined with other metals with which it does not conventionally alloy, powder metallurgy procedures may be employed to combine the characteristics of silver with the other metals in a manner in which true alloys cannot duplicate. Moreover, the chemical characteristics of the metal remain unchanged in powder metallurgy combinations. The electrical conductivity of the silver in powder metallurgy combinations is unchanged, so that the resulting conductivity may be only moderately less than than of the pure silver.
In the past, graphite and silver have been combined, by powder metallurgy techniques. The most frequently used composition is 95% silver and 5% graphite, although graphite combinations ranging from 0.25 to 90% with the remainder silver have been used. The advantage of graphite is that it prevents welding. However, silver graphite combinations are soft compared to other types of graphite materials and electrical and mechanical erosion is more rapid. Moreover, the silver graphite combinations exhibit inferior wear resistance though offering better protection against welding.